In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (AMLCD) devices. In the latter, a regular array of thin-film transistors (TFT) is fabricated on an appropriate transparent substrate such that the TFTs serve as integration regions and pixel regions.
Semiconductor films can be processed using excimer laser annealing (ELA), also known as line beam ELA, in which a region of the film is irradiated by an excimer laser to partially melt the film and then crystallized. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. ELA produces homogeneous small grained polycrystalline films; however, the method often suffers from microstructural non-uniformities which can be caused by pulse to pulse energy density fluctuations and/or non-uniform beam intensity profiles. In addition, it may take approximately 200 second to 600 seconds to completely process the semiconductor film sample using the ELA techniques, without even taking into consideration the time it takes to load and unload such sample.
Sequential lateral solidification (SLS) using an excimer laser is one method that has been used to form high quality polycrystalline films having large and uniform grains. A large-grained polycrystalline film can exhibit enhanced switching characteristics because the reduced number of grain boundaries in the direction of electron flow provides higher electron mobility. SLS processing also provides controlled grain boundary location. U.S. Pat. Nos. 6,322,625 and 6,368,945 issued to Dr. James Im, and U.S. patent application Ser. Nos. 09/390,535 and 09/390,537, the entire disclosures of which are incorporated herein by reference, and which are assigned to the common assignee of the present application, describe such SLS systems and processes.
In an SLS process, an initially amorphous (or small grain polycrystalline) silicon film is irradiated by a very narrow laser beamlet, e.g., laser beam pulse. The beamlet is formed by passing a laser beam pulse through a slotted mask, which is projected onto the surface of the silicon film. The beamlet melts the amorphous silicon and, upon cooling, the amorphous silicon film recrystallizes to form one or more crystals. The crystals grow primarily inward from edges of the irradiated area toward the center. After an initial beamlet has crystallized a portion of the amorphous silicon, a second beamlet is directed at the silicon film at a location less than the lateral growth length from the previous beamlet. Translating a small amount at a time, followed by irradiating the silicon film, promotes crystal grains to grow laterally from the crystal seeds of the polycrystalline silicon material formed in the previous step. As a result of this lateral growth, the crystals produced tend to attain high quality along the direction of the advancing beamlet. The elongated crystal grains are separated by grain boundaries that run approximately parallel to the long grain axes, which are generally perpendicular to the length of the narrow beamlet. See FIG. 6 for an example of crystals grown according to this method. One of the benefits of these SLS techniques is that the semiconductor film sample and/or sections thereof can be processed (e.g., crystallized) much faster that it would take for the processing the semiconductor film by the conventional ELA techniques. Typically, the processing/crystallization time of the semiconductor film sample depends on the type of the substrates, as well as other factors. For example, it is possible to completely process/crystallize the semiconductor film using the SLS techniques in approximately 50 to 100 seconds not considering the loading and unloading times of such samples.
When polycrystalline material is used to fabricate electronic devices, the total resistance to carrier transport is affected by the combination of barriers that a carrier has to cross as it travels under the influence of a given potential. Due to the additional number of grain boundaries that are crossed when the carrier travels in a direction perpendicular to the long grain axes of the polycrystalline material or when a carrier travels across a larger number of small grains, the carrier will experience higher resistance as compared to the carrier traveling parallel to long grain axes. Therefore, the performance of devices fabricated on polycrystalline films formed using SLS, such as TFTs, will depend upon the crystalline quality and crystalline orientation of the TFT channel relative to the long grain axes, which corresponds to the main growth direction.
In order to uniformly process the semiconductor films, it is important for the beam pulse to be stable. Thus, to achieve the optimal stability, it is preferable to pulse or fire the beam constantly, i.e., without stopping the pulsing of the beam. Such stability may be reduced or compromised when the pulsed beams are turned off or shut down, and then restarted. However, when the semiconductor sample is loaded and/or unloaded from a stage, the pulsed beam would be turned off, and then turned back on when the semiconductor sample to be processed was positioned at the designated location on the stage. The time for loading and unloading is generally referred to as a “transfer time.” The transfer time for unloading the processed sample from the stage, and then loading another to-be-processed sample on the stage is generally the same for the ELA techniques and the SLS techniques. Such transfer time can be between 50 and 100 seconds.
In addition, the costs associated with processing semiconductor samples are generally correlated with the number of pulses emitted by the beam source. In this manner, a “price per shot/pulse” is established. If the beam source is not shut down (i.e., still emit the beam pulses) when the next semiconductor sample is loaded unto the stage, or unloaded from the stage, the number of such irradiations by the beam source when the sample is not being irradiated by the beam pulse and corresponding time therefore is also taken into consideration for determining the price per shot. For example, when utilizing the SLS techniques, the time of the irradiation, solidification and crystallization of the semiconductor sample is relatively short as compared to the sample processing time using the ELA techniques. In such case, approximately half of the beam pulses are not directed at the sample since such samples are being either loaded into the stage or unloaded from the stage. Therefore, the beam pulses that are not impinging the samples are wasted.
Accordingly, it is preferable to reduce the price per shot, without stopping the emission of the beam pulses. It is also preferable to be able to process two or more semiconductor samples at the same time, without the need to stop or delay the emission of the laser beam pulses generated by the laser source until the samples are loaded on the respective stages.